Re: max_slot_wal_keep_size and wal_keep_segments

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: max_slot_wal_keep_size and wal_keep_segments
Date: 2020-07-08 16:20:31
Message-ID: 20200708162031.GA1716@alvherre.pgsql
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On 2020-Jul-09, Fujii Masao wrote:

> I attached the patch that renames wal_keep_segments to wal_keep_size.

Looks good in a quick once-over. Just two small wording comments:

> <para>
> Independently of <varname>max_wal_size</varname>,
> - <xref linkend="guc-wal-keep-segments"/> + 1 most recent WAL files are
> + most recent <xref linkend="guc-wal-keep-size"/> megabytes
> + WAL files plus one WAL file are
> kept at all times. Also, if WAL archiving is used, old segments can not be
> removed or recycled until they are archived. If WAL archiving cannot keep up
> with the pace that WAL is generated, or if <varname>archive_command</varname>

This reads a little strange to me. Maybe "the N most recent megabytes
plus ..."

> /* determine how many segments slots can be kept by slots ... */
> - keepSegs = XLogMBVarToSegs(max_slot_wal_keep_size_mb, wal_segment_size);
> - /* ... and override by wal_keep_segments as needed */
> - keepSegs = Max(keepSegs, wal_keep_segments);
> + slotKeepSegs = XLogMBVarToSegs(max_slot_wal_keep_size_mb, wal_segment_size);
> + /* ... and override by wal_keep_size as needed */
> + keepSegs = XLogMBVarToSegs(wal_keep_size_mb, wal_segment_size);

Since you change the way these two variables are used, I would not say
"override" in the above comment, nor keep the ellipses; perhaps just
change them to "determine how many segments can be kept by slots" and
"ditto for wal_keep_size".

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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